Regulator

ABSTRACT

To provide a regulator for restraining a variation in a frequency band and having a transient response characteristic which does not depend upon load current, by generating current in proportion to load current by a load current detecting transistor connected in parallel with an output driver transistor for supplying current to a load and changing a resistance value of a variable resistance portion by the current, a frequency of a zero point for phase compensation is varied and by varying the frequency of the zero point for phase compensation in accordance with the load current, the variation in the frequency band of the regulator is restrained without depending upon the load current and the transient response characteristic is improved.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to phase compensation for providinga transient response characteristic which does not depend on loadcurrent of a regulator.

[0003] 2. Description of the Related Art

[0004]FIG. 4 shows a constitution of a conventional regulator. Areference voltage power supply 201 supplies constant voltage Vref to aninverted input terminal of a transconductance amplifier 202. An outputof the transconductance amplifier 202 is connected to the gate of a PMOSoutput driver transistor 204 and a phase compensating RC network 203constituted by a resistor 208 and a capacitor 209. The source of thePMOS output driver transistor 204 is connected to an input terminal INand the drain is connected to an output terminal OUT. The outputterminal OUT is connected with a load resistor 207 and a capacitor 206and a voltage dividing circuit 205 constituted by resistors 210 and 211.The voltage dividing circuit 205 supplies voltage produced by dividingoutput voltage VOUT to a noninverting input terminal of thetransconductance amplifier.

[0005] When a resistance value of the resistor 208 constituting thephase compensation RC network 203 is designated by notation R208 and acapacitance value of the capacitor 209 is designated by notation C209,frequency fz of a zero point for phase compensation constituted by R208and C209, is calculated by the following equation. $\begin{matrix}{{fz} = \frac{1}{2{\pi \cdot {R208} \cdot {C209}}}} & (1)\end{matrix}$

[0006] When a resistance value of the load resistor 207 is designated bynotation R207 and a capacitance value of the load capacitor 206 isdesignated by notation C206, frequency fp of a pole constituted therebyis calculated by the following equation. $\begin{matrix}{{fp} = \frac{1}{2{\pi \cdot {R207} \cdot {C206}}}} & (2)\end{matrix}$

[0007] As is apparent from Equation (2), in accordance with a variationin the load resistor 207, the frequency fp of the pole is also changed.Meanwhile, as is apparent from Equation (1), the frequency fz of thezero point for phase compensation is a fixed value.

[0008] When load current is large, the load resistor 207 becomes smalland accordingly, by Equation (2), the frequency fp of the pole is movedto a high frequency side. Further, when the load current is small, theload resistor 207 becomes large and accordingly, by Equation (2), thefrequency fp of the pole is moved to a low frequency side. FIG. 5 showsfrequency characteristics of the regulator when the load current islarge and when the load current is small.

[0009] As shown by FIG. 5, when the load current is large, unity gainfrequency at which voltage gain of the regulator becomes 1, becomeshigh, conversely, when the load current is small, the unity gainfrequency becomes low. When the unity gain frequency is changed by theload current in this way, the transient response characteristic dependson the load current, which is not preferable. Particularly, when theload current is small, the unity gain frequency is low and accordingly,the transient response characteristic is deteriorated.

SUMMARY OF THE INVENTION

[0010] In order to resolve the above-described problem, according to theinvention, there is carried out an improvement in which by varying afrequency of a zero point for phase compensation in accordance with loadcurrent, a variation in a frequency band of a regulator is restrainedsuch that transient response does not depend upon the load current.

[0011] According to the invention, by generating current in proportionto load current by a load current detecting transistor connected inparallel with an output driver transistor for supplying current to aload and changing a resistance value of a variable resistance portion bythe current, a frequency of a zero point for phase compensation isvaried.

[0012] An improvement is carried out by varying the frequency of thezero point for phase compensation in accordance with the load current,thereby, a variation in a frequency band of a regulator is restrainedwithout depending upon the load current such that transient responsedoes not depend upon the load current.

BRIEF DESCRIPTION OF THE DRAWINGS

[0013]FIG. 1 is a circuit diagram of a regulator according to a firstembodiment of the invention;

[0014]FIG. 2 is a circuit diagram of a regulator according to a secondembodiment of the invention;

[0015]FIG. 3 is a diagram of frequency characteristics of the regulatoraccording to the second embodiment of the invention.

[0016]FIG. 4 is a circuit diagram of a regulator of a related art; and

[0017]FIG. 5 is a diagram of frequency characteristics of the regulatorof the related art.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0018] An explanation will be given of embodiments of the invention inreference to the drawings as follows.

[0019]FIG. 1 shows a regulator according to a first embodiment of theinvention. The reference voltage power supply 201 supplies the constantvoltage Vref to the inverted input terminal of the transconductanceamplifier 202. The output of the transconductance amplifier 202 isconnected to the gate of the PMOS output driver transistor 204, the gateof a load current detecting PMOS transistor 212 and a phase compensationRC network 203 constituted by the capacitor 209 and a variableresistance portion 215. The source of the PMOS output driver transistor204 is connected to the input terminal IN and the drain is connected tothe output terminal OUT. The output terminal OUT is connected with theload resistor 207, the capacitor 206 and the voltage dividing circuit205 constituted by the resistors 210 and 211. The voltage dividingcircuit 205 supplies voltage produced by dividing the output voltageVOUT to the noninverted input terminal of the transconductanceamplifier. The source of the load current detecting PMOS transistor 212is connected to the input terminal IN and the drain is connected to thevariable resistance portion 215.

[0020] When a gate width of the output driver transistor 204 isdesignated by notation W204, a gate length thereof is designated byL204, a gate width of the load current detecting transistor 212 isdesignated by W212 and a gate length thereof is designated by notationL212. Further, when drain current of the output driver transistor 204 isdesignated by notation I204 and drain current of the load currentdetecting transistor 212 is designated by notation I212, the followingrelationship is established. $\begin{matrix}{{I212} = {\frac{{W212}/{L212}}{{W204}/{L204}} \cdot {I204}}} & (3)\end{matrix}$

[0021] The drain current I204 of the output driver transistor 204 iscurrent supplied to load and accordingly, the drain current I212 of theload current detecting transistor 212 becomes current in proportion tothe load current and the proportional coefficient is given from Equation(3) as follows. $\begin{matrix}\frac{{W212}/{L212}}{{W204}/{L204}} & (4)\end{matrix}$

[0022] An arbitrary proportional coefficient can be set by pertinentlyadjusting gate sizes of the transistors 204 and 212.

[0023] In accordance with the Equation (3), the drain current I212 inproportion to the load current, outputted from the load currentdetecting transistor 212 is inputted to the variable resistance portion215. The variable resistance portion 215 changes a resistance valuethereof in accordance with inputted current.

[0024]FIG. 2 shows an embodiment further specifying the variableresistance portion 215. The variable resistance portion 215 isconstituted by a resistor 213 and an NMOS transistor 214. By flowing thedrain current I212 outputted from the load current detecting transistor212 and in proportion to the load current and current I216 outputtedfrom a constant current source 216, in the resistor 213, voltage isgenerated across both ends of the resistor 213. By the voltage generatedacross the both ends of the resistor 213, ON resistance of the NMOStransistor 214 is changed. Further, the constant current source 216operates such that the NMOS transistor 214 is not brought into anonconductive state even when the drain current I212 of the load currentdetecting transistor 212 becomes null.

[0025] As described above, ON resistance of the NMOS transistor 214operating as phase compensation resistor is changed in accordance withthe load current and accordingly, from Equation (1), the frequency fz ofthe zero point for phase compensation is also changed. The frequencycharacteristics of the regulator become as shown by FIG. 3 and even whenthe load current is changed, by restraining a variation in the unitygain frequency, the frequency characteristic of the regulator isimproved such that transient response does not depend upon the loadcurrent.

[0026] According to the invention, by generating current in proportionto the load current by the load current detecting transistor connectedin parallel with the output driver transistor for supplying current tothe load and changing the resistance value of the variable resistanceportion by the current, the frequency of the zero point for phasecompensation is varied.

What is claimed is:
 1. A circuit characterized in providing a transientresponse characteristic which does not depend upon load current byrestraining a variation in a frequency band of a regulator by loadcurrent by changing a frequency of a zero point for phase compensationby changing a resistance value of a phase compensation RC network inaccordance with the load current in the regulator.
 2. A regulatorcomprising: a load current detecting transistor connected in parallelwith an output driver transistor for supplying current to a load; aphase compensation RC network connected to an output end of the outputdriver transistor; and a variable resistor of the phase compensation RCnetwork connected to an output end of the load current detectingtransistor.